Mask ROM and fabrication thereof

ABSTRACT

A Mask ROM and a method for fabricating the same are described. The Mask ROM comprises a substrate, a plurality of gates on the substrate, a gate oxide layer between the gates and the substrate, a plurality of buried bit lines in the substrate between the gates, an insulator on the buried bit lines and between the gates, a plurality of word lines each disposed over a row of gates perpendicular to the buried bit lines, and a coding layer between the word lines and the gates.

BACKGROUND OF INVENTION

[0001] 1. Field of Invention

[0002] The present invention relates to a memory structure and a methodfor fabricating the same. More particularly, the present inventionrelates to a structure of a mask programmable read-only memory (MaskROM) and a method for fabricating the same.

[0003] 2. Description of Related Art

[0004] A Mask ROM generally comprises a substrate, a plurality of buriedbit lines in the substrate and a plurality of word lines crossing overthe buried bit lines, wherein the substrate under the word lines andbetween the buried bit lines serves as the channel regions of the memorycells. A method for programming a Mask ROM comprises implanting ionsinto the channel regions of selected memory cells to raise theirthreshold voltages, which is called a coding implantation. The data(0/1) stored in a memory cell is dependent on the presence/absence ofimplanted dopants in the channel region.

[0005] In a conventional coding process of a Mask ROM, a photoresistlayer is formed on the substrate and then patterned to form codingwindows over the channel regions of selected memory cells. An ionimplantation is then performed using the photoresist layer as a mask todope the selected channel regions. However, since the coding windows donot distribute evenly and there must be some regions with dense codingwindow patterns (dense regions) and some with isolated coding windowpatterns (sparse regions) on the coding photo mask, the criticaldimensions (CD) of the coding windows are not uniform. It is because theoptical proximity effect (OPE) for the dense regions is stronger thanthat for the isolated regions, and the light intensity through the denseregions therefore is higher than that through the sparse regions. The CDdeviations of coding windows cause misalignments of the codingimplantation, which may results in severe coding errors to lower thereliability of the Mask ROM product.

[0006] To prevent CD deviations over the sparse regions and the denseregions, quite a few methods are proposed based on the use of phaseshift masks (PSM) or on optical proximity correction (OPC) techniques.The OPC method forms assistant patterns on the photo mask to compensatethe CD deviations caused by the optical proximity effect (OPE). However,the two methods both need to design special patterns on the photo masks,so the fabrication of the photo masks are time-consuming, expensive anddifficult. Moreover, it is not easy to debug the patterns on such aphoto mask after the photo mask is fabricated.

[0007] Moreover, a Mask ROM coding implantation is usually performedwith boron ions in the prior art. However, the boron dopants implantedinto the selected channel regions tend to diffuse laterally to theadjacent buried bit lines, while the boron ions may even be implantedinto a portion of the adjacent buried bit lines because of themisalignments or CD deviations of the coding mask. Therefore, the dopantconcentrations of the buried bit lines are lowered to cause higherresistance and smaller electric current.

SUMMARY OF INVENTION

[0008] Accordingly, this invention provides a Mask ROM and thefabrication thereof to prevent diffusion of the implanted coding dopantsinto the buried bit lines and thereby avoid the current in the buriedbit lines from decreasing.

[0009] This invention also provides a Mask ROM and the fabricationthereof to improve the coding accuracy without using phase shift masks(PSM) or optical proximity correction (OPC), so as to reduce the costfor fabricating the photo mask.

[0010] A Mask ROM of this invention comprises a substrate, a gatedielectric layer, a plurality of gates, a plurality of buried bit lines,an insulator, a plurality of word lines and a coding layer. The gatesare disposed over the substrate, and the gate dielectric layer isdisposed between the substrate and the gates. The buried bit lines arelocated in the substrate between the gates, and the insulator isdisposed on the buried bit lines and between the gates. Each word lineis disposed over a row of gates and the insulator perpendicular to theburied bit lines. The coding layer is disposed between the word linesand the gates to constitute a plurality of memory cells, and comprises amaterial such as semiconductor. Some memory cells are implanted withcoding ions and are in a logic state of 1 (or 0), and the other memorycells are not implanted with coding ions and are in a logic state of 0(or 1).

[0011] A method for fabricating a Mask ROM of this invention isdescribed as follows. A gate dielectric layer is formed on a substrate,and then a strip conductive structure is formed on the gate dielectriclayer. An ion implantation is performed using the strip conductivestructure as a mask to form a buried bit line in the substrate besidethe strip conductive structure. Thereafter, the strip conductivestructure is patterned perpendicular to the buried bit line to form aplurality of gates. An insulator is formed between the gates by, forexample, forming an insulating layer on the substrate covering the gatesand then performing a CMP process or an etching-back process to remove aportion of the insulating layer until the gates are exposed. Then, amaterial layer and a conductive layer are sequentially formed over thegates and the insulator, wherein the material layer comprises a materialsuch as semiconductor. The conductive layer and the material layer aresequentially patterned to form a word line over the gates perpendicularto the buried bit line and a coding layer under the word line, whereinthe coding layer constitutes a plurality of memory cells. A coding maskis formed on the substrate. An implantation is then performed using thecoding mask as a mask to dope selected memory cells, wherein theselected memory cells are in a logic state of 1 (or 0), and the othermemory cells are in a logic state of 0 (or 1).

[0012] Since the coding ions are implanted into the coding layer betweenthe gates and the word line, the dopants do not diffuse to the buriedbit lines to decrease the electric current in the buried bit lines.Meanwhile, the buried bit lines are not affected even with misalignmentsor CD deviations of the coding mask, so phase shift masks (PSM) oroptical proximity correction (OPC) procedures are not required.

[0013] Accordingly, this invention is capable of increasing the marginof the Mask ROM coding process.

[0014] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary, andare intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

[0015] The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

[0016] FIGS. 1A-1D illustrate a process flow of fabricating a Mask ROMaccording to a preferred embodiment of this invention in a perspectiveview.

DETAILED DESCRIPTION

[0017] FIGS. 1A-1D illustrate a process flow of fabricating a Mask ROMaccording to a preferred embodiment of this invention in a perspectiveview.

[0018] Refer to FIG. 1A, a substrate 100 is provided, and then a gateoxide layer 102 and a plurality of strip conductive structures 104 areformed on the substrate 100, wherein the strip conductive structures 104comprise a material such as polysilicon. The method for forming the gateoxide layer 102 and the strip conductive structures 104 may be the onedescribed below. A thin oxide layer (not shown) and a conductive layer(not shown) are sequentially formed on the substrate 100, and then theconductive layer and the thin oxide layer are patterned into the stripconductive structures 104 and the gate oxide layer 102, respectively.

[0019] Thereafter, an ion implantation is performed using the stripconductive structures 104 as a mask to form a plurality of buried bitlines 106 in the substrate 100 between the strip conductive structures104.

[0020] Refer to FIG. 1B, the strip conductive structures 104 arepatterned perpendicular to the buried bit lines 106 to form a pluralityof gates 104 a. An insulator 108 is formed on the buried bit lines 106and between the gates 104 a to electrically isolate the gates 104 a fromeach other and from the buried bit lines 106. The insulator 108 formedby, for example, forming an insulating layer (not shown) on thesubstrate covering the gates 104 a and then performing a CMP process oran etching-back process to remove a portion of the insulating layeruntil the gates 104 a are exposed.

[0021] Refer to FIG. 1C, a coding layer 110 and a plurality of wordlines 112 thereon are formed over the substrate 100, wherein each wordline 112 is located over a row of gates 104 a and the insulator 108.Meanwhile, the coding layer 110 between the word lines 112 and the gates104 a constitutes a plurality of memory cells. The word lines 112 andthe coding layer 110 are formed by, for example, sequentially forming aglobal material layer (not shown) and a global conductive layer (notshown) covering the gates 104 a and the insulator 108, and thenpatterning the two perpendicular to the buried bit lines 106. The codinglayer 110 comprises a material such as a semiconductor material likeundoped polysilicon, and the word lines 112 comprise a material such asdoped polysilicon.

[0022] Refer to FIG. 1D, a coding mask 114 is formed over the substrate100 exposing portions of the word lines 112. An ion implantation 116 isperformed using the coding mask 114 as a mask to implant coding ions 118into selected memory cells in the coding layer 110. The selected memorycells implanted with coding ions 118 are in a logic state of 1 (or 0),and the memory cells not implanted with coding ions are in a logic stateof 0 (or 1). The coding ions 118 used to dope the coding layers 110 arepreferably PH₃ ions.

[0023] It is noted that the phosphorous dopants converted from PH₃ ionsdo not diffuse easily to cause coding errors in the preferred embodimentof this invention, so PH₃ ions are more preferable in the implantationprocess as compared with boron ions used in the prior art.

[0024] Refer to FIG. 1D, the Mask ROM according to the preferredembodiment of this invention comprises a substrate 100, a gate oxidelayer 102, a plurality of gates 104 a, a plurality of buried bit lines106, an insulator 108, a plurality of word lines 112 and a coding layer110. The gates 104 a are disposed on the substrate 100, and the gateoxide layer 102 is disposed between the substrate 100 and the gates 104a. The buried bit lines 106 are located in the substrate 100 between thegates 104 a, and the insulator 108 is disposed on the buried bit lines106 and between the gates 104 a. Each word line 112 is disposed over arow of gates 104 a and the insulator 108 perpendicular to the buried bitlines 106. The coding layer 110 is disposed between the word lines 112and the gates 104 to constitute a plurality of memory cells. The memorycells implanted with coding ions 118 are in a logic state of 1 (or 0),and the memory cells not implanted with coding ions are in a logic stateof 0 (or 1).

[0025] In the preferred embodiment, the coding ions are not implantedinto selected channel regions between the buried bit lines 106, but areimplanted into selected regions in the coding layer 110 on the gates 104a. Therefore, the dopants do not diffuse to the buried bit lines todecrease the electric current in the buried bit lines. Meanwhile, amisalignment to the selected memory cells in the coding layer 110 can betolerated since implanting ions into the coding layer 110 on theinsulator 108 does not affect the coding accuracy. Accordingly, thisinvention is capable of increasing the margin of a Mask ROM codingprocess. Furthermore, since a precise alignment to the selected memorycells is not required, the CD deviations of coding windows over thedense regions and the sparse regions do not affect the coding accuracy.Therefore, the yield of the Mask ROM can be improved.

[0026] In summary, with the Mask ROM structure of this invention and themethod for fabricating the same, the dopants do not diffuse to theburied bit lines to decrease the electric current in the buried bitlines. Meanwhile, since the buried bit lines are not affected even withthe misalignments or CD deviations of the coding mask layer, phase shiftmasks (PSM) or optical proximity correction (OPC) procedures are notrequired. Accordingly, this invention is capable of increasing themargin of the Mask ROM coding process.

[0027] It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncovers modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A Mask ROM, comprising:a substrates;a plurality of gates on thesubstrate; a gate oxide layer between the gates and the substrate;aplurality of buried bit lines in the substrate between the gates;aninsulator on the buried bit lines and between the gates;a plurality ofword lines over the substrate perpendicular to the buried bit lines,wherein each word line is located over a row of gates and the insulator;anda coding layer between the word lines and the gates constituting aplurality of memory cells, wherein some memory cells are implanted withcoding ions and are in a logic state of 1 (or 0), and the other memorycells are not implanted with coding ions and are in a logic state of 0(or 1).
 2. The Mask ROM of claim 1, wherein the coding layer comprises asemiconductor material.
 3. The Mask ROM of claim 2, wherein thesemiconductor material includes undoped polysilicon.
 4. The Mask ROM ofclaim 1, wherein the word lines comprise doped polysilicon.
 5. The MaskROM of claim 1, wherein the coding ions comprise PH₃ ions.
 6. The MaskROM of claim 1, wherein the insulator comprises silicon oxide.
 7. Amethod for fabricating a Mask ROM, comprising:forming a gate oxide layeron the substrate;forming a strip conductive structure on the gate oxidelayer;forming a buried bit line in the substrate beside the stripconductive structure;patterning the strip conductive structureperpendicular to the buried bit line to form a plurality ofgates;forming an insulator on the buried bit lines and between thegates;forming a coding layer and a word lines thereon over the substrateperpendicular to the buried bit line, wherein the word line is formedover the gates and the insulator, and the coding layer constitutes aplurality of memory cells;forming a coding mask over the substrate;andperforming an implantation using the coding mask as a mask to dopeselected memory cells, wherein the selected memory cells are in a logicstate of 1 (or 0), and the other memory cells are in a logic state of 0(or 1).
 8. The method of claim 7, wherein the coding layer comprises asemiconductor material.
 9. The method of claim 8, wherein thesemiconductor material includes undoped polysilicon.
 10. The method ofclaim 7, wherein the word line comprises doped polysilicon.
 11. Themethod of claim 7, wherein the coding ions comprise PH₃ ions.
 12. Themethod of claim 7, wherein forming the insulator comprises:forming aninsulating layer on the substrate covering the gates; andperforming anetching-back process or a chemical mechanical polishing (CMP) process toremove a portion of the insulating layer until the gates are exposed.13. The method of claim 7, wherein the insulator comprises siliconoxide.
 14. The method of claim 7, wherein forming the coding layer andthe word line comprises:forming a material layer on the gates and theinsulator;forming a conductive layer on the material layer;andpatterning the conductive layer and the material layer to form theword line perpendicular to the buried bit lines and, the coding layerunder the word line.
 15. The method of claim 7, wherein forming theburied bit line comprises performing an ion implantation using the stripconductive structure as a mask.